The invention discloses one-time-coating enamel and a preparation method thereof. The one-time-coating enamel comprises the following components in part by weight: 42 to 45 parts of silica, 4 to 6 parts of aluminum oxide, 7 to 9 parts of sodium oxide, 2 to 3 parts of potassium oxide, 2 to 3 parts of lithium oxide, 17 to 19 parts of diboron trioxide, 5 to 6 parts of titanium dioxide, 1 to 2 parts of calcium oxide, 3 to 4 parts of zirconium dioxide, 1 to 2 parts of sodium hexafluoroaluminate, 2 to 3 parts of strontium oxide, 3 to 4 parts of phosphorus pentoxide, 4 to 5 parts of cobalt oxide, 4 to 5 parts of nickel oxide, 4 to 5 parts of ferric oxide, 1 to 3 parts of potassium nitrate, 1 to 3 parts of sodium nitrate, 1 to 3 parts of boric acid, 1 to 3 parts of lithium fluoride and 1 to 3 parts of strontium carbonate. The preparation method comprises the following steps of: weighing all the components respectively; uniformly mixing the cobalt oxide, the nickel oxide and the nitrate; fully stirring all materials till the uniformity is more than or equal to 98 percent; smelting the mixture in a smelting furnace at the temperature of between 1,200 and 1,300 DEG C, taking a thread sample out and discharging the sample, wherein each meter of the sample is divided into 2 to 3 segments; pressing an enamel tablet by a tablet press till the thickness is less than 2mm; and cooling and smashing the obtained product. When the one-time-coating enamel is coated and enameled on the surface of steel sheets and cast irons, the bottom enamel unifying effect can be achieved at one time, so that the technical problem of one-time enameling is solved.